1. Field of the Invention
The present invention relates to a method of exposing a target by means of a plurality of beamlets, in particular by means of a charged particle multi-beamlet system, and to a computer readable medium for performing the methods when executed by a processor.
2. Description of the Related Art
Systems using a black and white writing strategy, i.e. an “on” and “off” writing strategy, are widely known in the art. They may use, for example, laser beams or charge particle beams, and may feature the use of direct writing means in maskless systems. By modulating the beam (or beams in multi-beam systems), individual grid cells in a rasterized virtual grid may be exposed or not exposed to write the desired pattern on to the target. Such beams are characterized by a so-called beam effect in the target surface, which is often described by a point spread function. The point spread function generally has a Gaussian distribution. The beam size is generally defined as the size of the distribution in which 50% of the beam energy is present.
A particular kind of charged particle beam based lithographic system is known from U.S. Pat. No. 6,897,458 assigned to the owner of the present invention, and involves a massive plurality of charged particle beamlets generated in a charged particle beam column for exposing a target. The charged particle beamlets are scanned over the target while being modulated. Additionally, the target may be capable of moving relative to the beams, for example in a direction transverse to the scanning direction of the beams. The modulation of the beamlets is performed on the basis of pattern data provided to the lithographic system. In the particular system described, the modulation is performed by blanking or blocking beamlets to effectively switch the beamlets on and off.
Exposing a target using this type of lithography system is achieved by the combination of relative movement of the target and modulation (e.g. timed “on” and “off” switching or blanking) of each charged particle beamlet by the blanker optics. A known method to expose a substrate with beamlets is a raster scan method. In order to accurately expose the target with an exposure pattern, the pattern data is rasterized. The target is positioned on a motor driven stage that is moved in a continuous motion. As the stage is moved, the beam is scanned in a direction substantially perpendicular to the stage motion. By supplying the rasterized pattern data to the system, timed so that the beamlets are modulated in synchronism with the beamlet deflection and stage motion, the pattern represented by the pattern data can be transposed as an exposure pattern onto the surface of the target. The rasterized pattern data corresponds to an exposure pattern on a virtual raster cell grid on the surface of the target.
Existing charged particle beam technology is suitable for lithography systems for relatively course patterning of images, for example to achieve critical dimensions of 90 nm and higher. However, a growing need exists for improved performance. It is desired to achieve considerably smaller critical dimensions, for example 22 nm, while maintaining sufficient wafer throughput, e.g. between 10 and 60 wafers per hour.
The total current of the beamlets required to achieve a particular throughput, e.g. in lithography defined as a particular number of wafers exposed per hour, depends on the required dose, the area of the wafer, and the overhead time, i.e. the time to move a new wafer into position for exposure. Among others, the required dose in these shot noise limited systems depends on the required feature size and uniformity, and beam energy.
For electron beam systems, the maximum single beam current is determined by the spot size. To obtain a good CD uniformity, the required spot size will limit the single beam current to much less than the current required to obtain a high throughput. Thus a large number of beamlets is required (typically more than 10,000 for a throughput of 10 wafers per hour). As the total current through one lens is limited by Coulomb interactions between electrons, the number of lenses in a high throughput system also needs to be large.
Increasing the current in the system, results in an increase of the total current on the target. At the same time, however, in order to maintain performance, the number of electrons impinging on the target surface per square critical dimension should be maintained constant.
However, designing a system to generate beamlets having a smaller spot size, considerably reduces the charged particle current that may be applied to the target by each beamlet. Irrespective of the brightness of the charged particle source used, the preceding requirements imply a considerably more than linear increase in the number of beamlets in a charged particle multi-beam system compared to the reduction in critical dimension at the same wafer throughput.
In order to obtain a high throughput using such a multi-beam system, a beamlet writing strategy is required to expose the desired pattern as efficiently as possible. The relative size of the beamlets and the lenses that focus the beamlets compared to the desired size of the virtual grid limits the possible arrangements of beamlets, so that the beamlets are separated by a sufficiently greater distance than the diameter of the lenses.
It is desired that the writing strategy does not require overly complex movement of the beamlets and the target. In this regard, a system providing for uniform deflection of all the beamlets rather than individual control of each beamlet's deflection is desired, particularly for a system having a very large number of beamlets. As a consequence, a simple raster scan of the beamlets as a group may be undesirable due to the incomplete exposure of the areas at the end of each scan of the group of beamlets. The larger the number of beamlets, the greater the effect of these areas of incomplete exposure will have on efficient utilization of the system. It is also desired that the writing strategy reduces the effects of unintended variations between the individual beamlets in a multi-beamlet system.
It is therefore desirable to use a method of exposing a target by means of a plurality of beamlets with an improved performance.